1. Field of the Invention
The present invention relates to a high-stability piezoelectric oscillator and, more particularly, to a high-stability piezoelectric oscillator that uses a Peltier element as a temperature control element for an inner constant temperature oven of a double constant temperature oven to set the temperature of the inner oven lower than the temperature of the outer oven.
2. Background Art
Because of its excellent frequency accuracy, frequency-temperature characteristics, frequency-aging characteristics, a high-stability crystal oscillator (OCXO) is in wide use ranging from a mobile radio base station to a high-precision measuring instrument. The high-stability crystal oscillator has a construction in which a quartz crystal resonator and an oscillation circuit are placed in a constant temperature oven to prevent the oscillation frequency from varying with ambient temperature. A high-stability crystal oscillator for higher precision application uses a doubly-rotated quartz crystal resonator, such as SC-cut or IT-cut quartz crystal resonator, and it is known in the art that the use of such a quartz crystal resonator provides a crystal oscillator whose stress-sensitivity and thermal shock resistance characteristics are more excellent than in the case of using an AT-cut quartz crystal resonator.
FIG. 3 is a graph showing reactance characteristics of near resonance point of the SC-cut quartz crystal resonator, the abscissa representing frequency and the ordinate representing reactance; the quartz crystal resonator is excited in three modes, that is, a thickness shear mode (C-mode) for the principal vibration at the lowest frequency among the three modes, a thickness twist mode (B-mode) and a thickness longitudinal mode (A-mode) at higher frequency than the thickness shear mode. Since the resonance frequency (f2) of the B-mode adjacent to the C-mode (the resonance frequency f1) of the principal vibration is about 9 to 10% higher than the C-mode resonance frequency f1, the oscillation circuit is so adapted as to prevent the occurrence of a so-called frequency jump phenomenon.
FIG. 4 is a graph showing the frequency-temperature characteristic of the SC-cut quartz crystal resonator (C-mode), which assumes a cubic curve with the inflection point temperature (Ti) at approximately 95° C., the abscissa representing temperature (° C.) and the ordinate a normalized frequency variation (Δf/f). The peak temperature Tp, which has a zero temperature coefficient at a temperature lower than the inflection point temperature (Ti), is dependent on the second angle of rotation at which quartz crystal is cut, and the peak temperature Tp is set substantially in the range of 65° C. to 81° C. Accordingly, by setting the temperature in the constant temperature oven at a value close to the peak temperature Tp, it is possible to obtain a stable frequency.
For example, when the high-stability crystal oscillator is used over a temperature range from 0° C. to 50° C., provision is made to hold the temperature in the constant temperature oven, for example, at 70° C., 10° C. to 20° C. higher than the upper limit temperature 50° C. And, the use of the SC-cut quartz crystal resonator whose peak temperature Tp is approximately 70° C. ensures a stable oscillation frequency of the oscillator without being affected by ambient temperature.
FIG. 5 is a sectional view showing the construction of the high-stability crystal oscillator, wherein a printed board 21 having mounted thereon an SC-cut quartz crystal resonator 22, electronic parts 23 for oscillation use and a thermo-sensitive device 24 is housed in a constant temperature oven 25, and a heater 26 is wound around the constant temperature oven 25. A power supply terminal, an output terminal, and other terminals 27, 27 . . . extended from the printed board 21 pass in insulation through the constant temperature oven 25 and a base 28 by use of hermetic terminals or the like. The base 28 is covered with a case 29, their joints being sealed by soldering or the like. Voltage application to the terminal 27 causes a current flow through the heater 26, and the thermo-sensitive device and a control circuit operate to keep the temperature of the constant temperature oven 25 at a fixed value.
In recent years, there is a demand for a high-stability crystal oscillator that operates stably even under high ambient temperature conditions. For example, when the working temperature range is 0° C. to 85° C., the temperature of the constant temperature oven needs to be set at about 95° C. higher than the upper limit 85° C. of the ambient temperature.
However, electronic parts comprising the oscillation circuit are mostly rated up to approximately 85° C., and their use at higher temperatures is not guaranteed. There are also electronic parts for military and satellite use that can be used at higher temperatures, but they are extremely expensive. Furthermore, high-temperature operations also accelerate aging of the quartz crystal resonator, arousing the fear of a frequency shift or the like. Besides, the peak temperature Tp of the SC-cut quartz crystal resonator is 81° C. at the highest and cannot be set higher.
In view of the above, there is proposed a high-stability crystal oscillator using a Peltier element (an element utilizing a Peltier effect, a phenomenon that the application of current to the junction of different kinds of conductors (or semiconductors) causes generation or absorption of heat. The amount of heat generated or absorbed is in proportion to the current applied, and reversal of the direction of current reverses the generation and absorption of heat.) as disclosed in Japanese Laid-open application publication No. 3-104404. For example, when the working temperature range is 0° C. to 85° C., if the inside temperature of the constant temperature oven is set at 70° C., the Peltier element and the temperature control circuit operate to heat or cool the constant temperature oven to maintain it at 70° C. In this instance, an SC-cut quartz crystal resonator with Tp=70° C. can be used as the quartz crystal resonator.
FIG. 6 is a sectional view showing the construction of the high-stability crystal oscillator using the Peltier element, wherein a printed board 31 having mounted thereon an SC-cut quartz crystal resonator 32, electronic parts 33 for oscillation use, and a thermo-sensitive device 34 is housed in a constant temperature oven 35, and a Peltier element 36 is mounted on the constant temperature oven 35. A power supply terminal, an output terminal, and other terminals 37, 37 . . . extended from the printed board 31 pass in insulation through the constant temperature oven 35 and a base 38 by use of hermetic terminals or the like. The base 38 is covered with a case 39, their joints being sealed by soldering or the like. By applying current to the Peltier element 36 via the power supply terminal 37, the constant temperature oven 35 can be held at a predetermined temperature, for example, at 70° C.
In the high-stability crystal oscillator using the Peltier element as shown in FIG. 6, however, it is very difficult to keep the inside temperature of the oven at 70° C. when an ambient temperature is about 70° C. That is, the inside temperature of the oven is controlled by the current supply to the Peltier element 36, however, no current is applied to the Peltier element when the ambient temperature is about 70° C. or so. Therefore, it is difficult to control the inside temperature under the condition that the current supply is almost zero or very small. As is well known in the art, the temperature of the constant temperature oven becomes stable and easy to control by setting it at least 10° C. to 20° C. higher than the ambient temperature.
The present invention is intended to solve the above-mentioned problems, and has for its object to provide a high-stability crystal oscillator that operates stably even when the required temperature range is as wide as 0° C. to 85° C. and minimizes deterioration of the quartz crystal resonator and electronic parts used due to aging.